The present invention relates to electrical discharge machining processes for satinizing by electrical discharge the outer surface of a cylindrical work to give a satiny finish of a predetermined surface roughness, and more particularly the invention relates to an improved electrical discharge machining process and apparatus designed to prevent the occurrence of stepped portions on the surface of a roll during the initial period of the electrical discharge machining.
In the past, the method of throwing hard metal particles, e.g., shot or grit, at the polished surface of strip rolling rolls, particularly cold rolling rolls, to produce impressions on the roll surface, has been used as a means of giving a satinlike finish to the surface of such roll, and recently attempts have been made to replace this type of process by electrical discharge machining. As is well known, the electrical discharge machining is a process in which an insulating liquid, e.g., kerosene, is placed in the small discharge gap between an electrode and a work piece and a pulse voltage is periodically applied across the electrode and the work piece, thus working the surface of the work piece. By repeatedly carrying out this electrical discharge machining at the surface of a roll while rotating the roll in the circumferential direction and also gradually moving the electrode in the lengthwise direction of the axis of rotation of the roll, it is possible to continuously give a spiral satiny finish to the roll surface and thereby to produce spark eroded impressions all over the roll surface. This is the known method of giving a uniform satiny finish to the surface of a roll surface by means of electrical discharge machining. The satinfinished surface produced in this way has many advantages, that is, not only the roughness is more marked and the shape is in excellent trim as compared with the mechanical impressions produced by the throwing of metal particles, but also the shape is not affected by the manufacturing method, hardness, etc., of the roll, the metal structure in the roll surface being hardened by the electrical discharge, thus making the roll best suited for rolling purposes and so on.
With the more recent roll machining processes, the machining is performed by using a multi-piece electrode to reduce the working time, and as a result where an extremely large number of split electrodes are used, in order to prevent deterioration in the split electrode working efficiency and ensure mechanical structural stability, a method has been proposed in which a plurality of head columns (a head and a column are considered as a unit and the unit is hereinafter referred to as a head column) are arranged to face a roll, and a plurality of split electrodes are mounted on each of the head columns, whereby the plurality of head columns are moved along the axis of rotation of the roll during the operation to effect the machining of the roll.
A disadvantage of this method is that, since the feeding of the electrode spindles at each head column is controlled independently, the working at each head column is started at a different time in some cases, with the result that if the movement of a plurality of the head columns along the axis of rotation of the roll is initiated at the instant that the working is started at one of the head columns, the working rate around the working initiating position of each of the plurality of the other head columns is decreased, and consequently the radial length of the roll at these portions increases as compared with that at the other portions. On the contrary, if all of the head columns are held in their initial positions until the working is started at every head column, as compared with the working rate at the head column which is brought into operation finally, the working rate at the head column which was put into operation earlier is increased, thus decreasing the radial length of the roll. In particular, this effect will be more marked in cases where the working conditions are preset to produce a surface of a greater surface roughness, thus causing stepped portions on the roll surface. These stepped portions will be a serious defect in the case of a roll used for example in the final rolling of sheet steel, and any roll worked in this way cannot be put into use. Such stepped portions can be checked by rubbing the roll surface with an oil-stone.